A five-month-old baby killed by his mother suffered severe brain injuries, bruising to his head and neck, and had meth in his system, a Perth court has heard.

Melissa Louise Bulloch, 27, a mother of three, has pleaded guilty to the manslaughter of her son Lachlan, who was found dead at his Hamilton Hill home, southwest of Perth, in January 2014.

Prosecutor Carmel Barbagallo told the WA Supreme Court during a sentencing hearing on Thursday that Bulloch's abuse of the baby in the months leading up to his death included blowing cannabis smoke into his face, leaving him in a car, and shaking and swearing at him.

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In the months leading up to his death, five-month-old Lachlan (pictured), who was killed by his mother in Perth in January 2014, was shaken and had cannabis smoke blown in his face, a court has heard

The court heard the Department for Child Protection was also involved with the family and police listened to Bulloch's phone calls and conversations after the baby's death.